Heat treatment for wrought zinc-aluminum alloys

ABSTRACT

The creep resistance and strength of wrought zinc-aluminum alloys having a zinc-aluminum eutectoid transformation is improved by a heat treatment comprising slow cooling the alloy from a temperature between its eutectoid and solidus temperatures to below its eutectoid temperature and subsequently rapidly cooling the alloy.

United States Patent 1191 Gervais et al.

1451 Jan. 28 1975 1 HEAT TREATMENT FOR WROUGHT ZINC-ALUMINUM ALLOYS [75] Inventors: Edouard Gervais, Montreal; Pierre Chollet, Pierrefonds, Quebec, both [2]] Appl. No.: 250,556

(56] References Cited UNITED STATES PATENTS 3,676,] l5 7/1972 Hare et al 75/l78 AM Primary Ii.\'aminerW. Stallard 1571 ABSTRACT The-creep resistance and strength of wrought zincaluminum alloys having a zine-aluminum eutectoid transformation is improved by a heat treatment com- Foreign Application Priority Data prising slow cooling the alloy from a temperature be- July 21, 1971 Canada 118721 tween its eutectoid and solidus temperatures to below its eutectoid temperature and subsequently rapidly [52] US. Cl. 148/115 R, 148/13 cooling the alloy. 1 [51] Int. Cl. ..C22f1/16 v [58] Field of Search 148/115 R, 13 14 Clam, 3 Draw F'gms CURVE AVERAGE COOLING RATE (C/M|N.)10.

2 1.42 1191 0.78 8 CURVE 5 3 1.07 0.85 0.12 L 300 4 0.66 0.6 0.47 11.1 s 0.46 m E 200 m D. E w 1..

cuRvE NQZA i- CURVE No.2 0 1 1 1 1 1 1 1 1 1 1 8 9 IO ll TIME IN HOURS PATENTEB JAN 2 8 I975 SHEET 3 BF 3 N O m w 90% 22K @2308 30 5 I III ooofim I 80 3 I 000 ooodm I 80 8 II' mismaais EI'IISNELL zuvwlnn HEAT TREATMENT FOR vvRououT ZINC-ALUMINUM ALLOYS This invention relates generally to a heat treatment for wrought'zinc-aluminum alloys.

More particularly, the present invention is a heat treatment for wrought zinc-aluminum alloys having a eutectoid transformation, which optionally contain copper, magnesium, or bismuth to improve the creep resistance and the ultimate tensile strength of the alloys.

The heat treatment of the present invention is especially applicable to zinc-aluminum alloys containing about 12% to about 30% (preferably about 18% to about 30%) aluminum, to about copper, 0% to about 1% magnesium, 0% to about 3% bismuth and the balance zinc except for incidental impurities.

Considerable research has been conducted on zincaluminum alloys of near eutectoid composition since it is acknowledged that such alloys have a variety of commercial applications. In the applicants US. Pat. application Ser. No. 108,199 filed Jan. 20, 1971, a heat treatment is disclosed for wrought zinc-aluminum alloys containing 18 to 30% aluminum, up to 3% copper, up to 0.1% magnesium, up to 0.1% lithium and the balance zinc apart from incidental impurities, wherein the physical properties of the alloy are improved by slow cooling the alloy to ambient or room temperature from a temperature above the eutectoid temperature and below the solidus temperature. A preferred embodiment of that invention involves slow cooling the alloy from about 380C. to room temperature.

Difficulties with zinc-aluminum alloys have often been encountered due to the high creep rates of such alloys. A principal advantage of the heat treatment of the present invention is that the creep resistance of such alloys is improved as compared to alloys heat treated according to the method described by the same inventors in US. Pat. application Ser. No. 108,199.

A further advantage is that alloys heat treated according to the present invention have a higher ultimate tensile strength and hardness than alloys heat treated according to the method described in U.S. Pat. application Ser. No. 108,199.

The heat treatment according to the present invention comprises homogenizing and slow cooling wrought zinc-aluminum alloys having a zinc-aluminum eutectoid transformation from above the eutectoid temperature and below the solidus or eutectic temperature to a temperature below the eutectoid temperature where eutectoid decomposition is substantially complete, and then rapidly cooling the alloy to room temperature. By interrupting the slow cooling heat treatment process described in US. Pat. application Ser. No. 108,199 at a temperature of about 250C. and then rapidly air cooling the alloy to room temperature, considerable improvement in the creep resistant properties is achieved as compared to an alloy which is slow cooled by furnace cooling to room temperature.

With reference to the preferred ranges of alloying constituents contemplated for treatment by the present invention it should be noted that the present heat treatment for a wrought zinc-aluminum alloy containing 12 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities comprises the steps of homog-' enizing and slow cooling the alloy from above its eutectoid temperature and below its solidus or eutectic temperature to a temperature below its eutectoid temperature whereat the eutectoid transformation is substantially complete, and rapidly cooling the alloy. The heat treatment according to the present invention also results in an increase in hardness and ultimate tensile strength as compared with alloys of the same composition slow cooled to room temperature. However, as will be seen from the data in the tables at the end of this disclosure, the improvement in creep resistance is a more significant feature of the present invention.

FIG. 1 is a series of cooling curves from 380C. for a zinc-aluminum alloy.

FIG. 2 is a creep deformation curve for a typical zincaluminum alloy.

FIG. 3 is a graph showing the effect of copper content and magnesium additions on the ultimate tensile strength of zinc 25% aluminum alloys heat treated according to the present invention and according to the method described in US. Pat. application Ser. No. 108,199.

According to the present invention, a heat treatment is provided for wrought zinc-aluminum alloys having a zinc-aluminum eutectoid transformation involving slow cooling the alloy from a temperature above its eutectoid temperature and below its solidus temperature to a temperature below the eutectoid temperature whereat the eutectoid transformation is substantially complete, and thereafter rapidly cooling said alloy.

The alloys investigated were produced by melting the elements in the desired proportions in an induction furnace and by semi-continuous casting using the controlled cooling method. The cast billets were extruded under lubrication at a temperature of 250C. and the extrusion product was then cold drawn.

The heat treatment procedure consisted of two stages:

STAGE I The alloy was slow cooled from a temperature above the eutectoid temperature and below the solidus or eutectic temperature after a brief period of homogenization. The homogenization temperature was the same temperature from which slow cooling is intended to commence. The average rate of cooling was preferably less than about 2.0C./min. (For alloys with less than about 18% aluminum the solidus temperature becomes the eutectic temperature).

STAGE II- The slow cooling process of Stage I was interrupted below the eutectoid temperature, where eutectoid decomposition is substantially completed, and the sample was rapidly cooled, usually in air.

In FIG. 1, Curve 2 shows the cooling rate of a sample within the range described in the above-mentioned United States patent application. The other curves represent various cooling rates which were used during the study. The plateau shown on the curves corresponds to the eutectoid transformation; the eutectoid transform ation starts at the beginning of the temperature arrest and is largely completed when the temperature decreases again. For the heat treatment to be fully effective, the sample should not be removed from the furnace before the end of the plateau.

An example of a cooling curve of Stage 11 according to the present invention is shown in FIG. 1 by the dotted line identified as Curve 2A. Curve 2A schematically represents air cooling from 250C. to room temperature.

Stage I of the two heat treatments is the same; for example, the specimens are slowly cooled from about 380 to about 250C. The improvement according to the present invention results from Stage II.

The exothermic eutectoid transformation results in a plateau of flattening of the cooling curve. The resumption of a normal cooling rate indicates that eutectoid transformation is substantially complete.

The tensile tests were carried out on a Tinius-Olsen tensile machine using 0.375 inch diameter samples at a cross head separation rate of 0.25 in./min. The percent elongation was determined over a 2 inch gauge length and the percent area reduction was determined from the minimum diameter of the sample at rupture.

The hardness of each sample was determined with a Vickers Hardness Tester; all measurements were made using a -kilogram load.

The creep test samples were designed in accordance with ASTM specification E.l39-58T. All creep tests were carried out under a constant load of 20,000 lb/in. and at a temperature of 68 1 lF.; the sample deformation over the two inch gauge length was recorded using an optical system. FIG. 2 shows a typical creep curve and defines the two criteria used to quantify the results. The first criterion corresponds to the well known inverse creep rate which is defined as the inverse of the slope of the straight portion (second stage creep); the units are day/%. The second criterion corresponds to the time required to reach 1% plastic deformation. I

Table I shows the effect of various heat treating procedures on the mechanical properties of a zinc base alloy containing 25% Al, 1% Cu and 0.05% Mg.

A comparison of Examples 1 and 2 shows that a sample which is furnace cooled from 380C. has greatly improved creep resistance (78 days/%) compared to a sample in the as extruded and drawn stage (2.5 days/%). The sample slow cooled from 380C. shows lower tensile properties and hardness.

A comparison between Examples 2 and 3 illustrates that a sample that has been slow cooled from 380C. to 250C. and then air cooled shows a further improvement in creep resistance (266 days/%) compared to a sample which has been slow cooled from 380C. without any interruption in the slow cooling. Example 3 with the interrupted cooling also shows some improvement in ultimate tensile strength and hardness over slow cooled Example 2.

Example 4 shows slow cooling from 290C. where the improvement in 'creep resistance (3.6 days/%) compared to untreated Example 1 is only slight.

Example 5 shows slow cooling from 290C. to 225C. followed by air cooling and a further improvement in creep resistance (17.9 days/%) compared to slow cooled Example 4.

The effect on mechanical properties of heating (ageing) the alloy for 3 hours at 250C. followed by air cooling (Example 6) has been added to Table I to demonstrate that it is important to go above the eutectoid temperature. Hence, simply heating a sample to 250C. and air cooling is not sufficient to develop a creep resistance comparable to those obtained when using the method of heat treating of the present invention.

From an examination and comparison of the Examples in Table I it is seen that the most improvement in creep resistance is obtained by slow cooling the alloy from near the solidus temperature to a temperature below the eutectoid temperature and then rapidly cooling the alloy to room temperature.

Although the examples of the present application are limited to tests conducted by cooling from 380C. and 290C. respectively, slow cooling from any temperature between the eutectoid temperature and the solidus temperature would produce satisfactory results. In the commercial use of the present invention slow cooling of the alloy should commence at a temperature that is sufficiently below the solidus temperature such that the solidus temperature would not accidentally be exceeded. Therefore, slow cooling from a temperature that is safely below the solidus temperature and above the eutectoid temperature would comprise the temperature range for use in the present invention.

Tests were undertaken to determine the effect of the air cooling temperature (Stage II) on the mechanical properties of a zinc alloy containing 25% Al, 1% Cu, 0.05% Mg which was first homogenized and slow cooled from either 380C. or 290C. Table II illustrates the results for samples slowly cooled from 380C. to various temperatures at which rapid cooling commences and compares them with the results obtained from the air cooling to room temperature from 380C. The data demonstrates the importance of waiting until the sample has undergone complete eutectoid transformation before air cooling is commenced if high creep resistance is to be obtained.

Table IIfurther demonstrates that if the temperature at which air cooling starts is too low a loss in creep resistance results. This table also shows that air cooling samples from temperatures below the eutectoid temperature gives superior hardness, creep and tensile strength properties than for a sample slowly cooled to room temperature.

Since the heat treatment consists of two stages, the sensitivity to the cooling rate of each stage was investigated to determine the effect on the mechanical properties.

Table III demonstrates the effect of the average cooling rate to 250C. on the mechanical properties of alloys slowly cooled from 380C to 250C. (Stage I) and thereafter air cooled to room temperature (Stage II). It shows that the alloys and heat treatment are quite insensitive to a cooling rate of Stage I within the range shown in Table III; the only significant property variation isan increasein ultimate tensile strength and hardness obtained with the fastest cooling rate.

Heating treating with cooling from 290C. is more sensitive to cooling rate. Table IV shows that the slower cooling rate to a temperature of 225C., although leading to a slight reduction of the ultimate tensile strength results in a significant increase of the creep resistance.

The effect of the cooling rate during the second stage of the heat treatment is shown in Table V from which it is apparent that the faster the cooling, the higher the ultimate tensile strength, and the lower the tensile ductility. In view of the practical considerations, air cooling is preferred for most applications of the present invention.

Since the dimensional and structural stability of the zinc-aluminum alloys is frequency of concern, tests were conducted to determine the performance of the material upon dry ageing. The test consists of heating the samples for 10 days at C. Ten days exposure at 95C. is considered the equivalent of ten years exposure at ambient temperature in regard to ageing stability according to the brochure entitled ILZRO 12: A New Zinc Gravity Cast Alloy published by the International Lead-Zinc Research Organization Inc., New

invention is effective over the whole range-of components investigated.

In addition to the heat treatment having a beneficial effect on alloys containing about 12% to about 30% aluminum, up to copper, up to 1% magnesium, York, Aprll 196 and up to 3% bismuth and the balance zinc it is antici- Table VI illustrates the tensile properties before and Pated that the heat ment of the present invention after dry ageing 2119500 for a zinc base 2 5% Al, 1% Cu should also have useful application for zinc-aluminum 005% Mg alloy heat treated according to the present alloys WhlCh have a eutectold phase transformation but invention. It demonstrates that the alloy was stable much are outslde )9 above range of Compositions after heat treatment and that ageing only slightly re- Such as alloys contafntmg 10 to 50% aluminum, and duced the ultimate tensile strength. The dimensional possfbly alloys Contammg p pp stability of the heat treated alloy was considered excel- It Preferable that homogemlatlo" take Place at a lent. temperature where the alloy is substantially all in the a I 15 phase, where it is possible to do so.

FIG. 3 and Table VII demonstrate the wide range of It can be appreciated from the foregoing that in its alloys to which the present invention is applicable and broad aspects the present invention involves a heat also illustrate the effect that varying amounts of the diftreatment for a wrought zinc-aluminum alloy having a ferent elements have on their tensile properties. In zinc-aluminum eutectoid transformation comprising Table VII, the properties of alloys slowly cooled from the steps of slow cooling the alloy from above its eutec- 380C. with and without rapid cooling from 250C. are toid temperature and below its solidus or eutectic temcompared. FIG. 3 further demonstrates the advantages perature to a temperature below the eutectoid temperof the two-stage heat treatment over slow cooling from ature whereat the eutectoid transformation is substan- 380C. to room temperature. It also shows, for both 7 tiaIIy complete, and subsequently rapidly cooling said methods of heat treating, that the ultimate tensile alloy. strength increases with the copper content. Table VII Generally, it can be readily seen that the present inshows that ductility decreases with increasing copper ention represents a significant advance in the art content. The alloys containing bismuth should prefera- Which Should Provide n g s d e efits to those bly contain magnesium in the proportions described in Segments of industry involved in the manufacture or our copending US. application Ser. No. 250,557. use of the alloys which may be treated according to the Table VII shows that the heat treatment of the present present invention.

TABLE I EFFECT OF METALLURGICAL CONDITION ON MECHANICAL PROPERTIES Nominal Composition: Zn 25% Al, 1% Cu, 0.05% Mg Tensile Properties Creep Tests at 20,000 lb./in. at 68F Example Metallurgical Condition Ultimate Hardness Time to Tensile Elon- Reduction V.H.N. Inverse Creep Rate 1'71 Plastic Strength, gation of Area 5-Kilogram 95% Confidence Limit Deformation,

(lb/in?) Load (days/% (days) 1 As extruded and drawn 62,600 29 70 139 2.6 $0.1 1.7 2 Furnace cooled from 380C 45,800 23 107 78 i 6 53 (Curve 2 FIG. 1 of United States Patent Application Serial No. 250,557) 3 Furnace cooled from 380C to 54,800 20 40 124 226 i 12 150 250C (Curve 2) then air cooled 4 Furnace cooled from 290C. 45,100 32 58 105 3 6 i 0.2 1.8

Ref. United States Patent Application Serial No. 250,557 5 Furnace cooled from 290C to 225C 50,200 31 55 112 17.9 1 8.7 then air cooled 6 3 hours at 250C and 55,200 25 57 N.A. 12.5 i 0.8 6.7

air cooled TAB LE 11 EFFECT OF AIR COOLING TEMPERATURE ON THE MECHANICAL PROPERTIES UPON SLOW COOLING FROM 380C Nominal Composition: Zn 25% Al, 1% Cu, 0.05% Mg Tensile Properties Creep Tests at 20,000 lb./in. at 68F Stage 11 Ultimate Hardness Air Cooling Tensile Elon- 70 Reduction V.H.N. Inverse Creep Rate i Time to 1% Temperature, C Strength, gation of Area 5-Kilogram Confidence Limit Plastic Deformation (lb/in?) Load (days/'70) (days) 380 64,600 17 40 143 45.8 i 6.0 33 Beginning of plateau 68,900 50 158 37 i 2 27 End of plateau 61,900 88 i 5 53 260 58,050 16 25 162 :10 82 250 54,800 20 40 124 226 i 12 150 225 51,950 23 47 200 48,800 24 50 113 119:9 91 175 47,100 24 54 107 N.A. N.A. 46,100 24 52 107 85 i 6 62 120 45,900 25 52 104 N.A. N.A.

TABLE III EFFECT OF COOLING RATE FROM 380 to 250C* ON THE MECHANICAL PROPERTIES OF AN ALLOY CONTAINING Zn, 25% Al. 1% Cu, 0.05% Mg Tensile Properties Creep Tests at 20,000 lb/in. at 68F Average Cooling Rate Ultimate Hardness from 380 to 250C, Tensile "7: Elon- 70 Reduction V.H.N. Inverse Creep Rate Time to 1% (C/min.) Strength gation of Area S-Kilogram 95% Confidence Limit Plastic Deformation (lb./in. Load (days/V1 (d Curve I 18 58,700 16 40 134 210: 8 I68 Curve 2 1.01 54,800 20 40 124 226 :12 150 Curve 4 6 52.700 20 41 122 249 i II 172 Curve 0.44 54.600 14 30 126 232 I I1 I86 Note: Subsequent cooling from 250C is air cooling.

TABLE IV EFFECT OF COOLING RATE FROM 290 to 225C* ON THE MECHANICAL PROPERTIES OF AN ALLOY CONTAINING Zn. 25% Al, 1% Cu, 0.05% Mg Tensile Properties Creep Tests at 20.000 lb./in. at 68F Average Cooling Rate Ultimate Hardness from 290 to 225C Tensile 71' Elon- Reduction V.H.N. Inverse Creep Rate i Time to 1% (C/min.) Strength gation of Area S-Kilogram 95% Confidence Limit Plastic Deformation (lb./in. Load (days/71) (days) Note: Subsequent cooling from 225C is air cooling.

TABLE V EFFECT OF COOLING RATE DURING STAGE 2 ON THE MECHANICAL PROPERTIES OF AN ALLOY CONTAINING Zn 25% Al, 1% Cu, 0.05% Mg Tensile Properties Ultimate Hardness Method of Average Cooling Rate Tensile Elon- Reduction V.H.N. Cooling from to Room Temperature Strength gation of Area S-Kilogram 250 to 50C (C/min.) (lb./in. Load Ice water quenched About 1400 56,100 17 25 124 Air cooled 19 54,800 20 40 124 Furnace cooled Curve 2 .22 45,800 23 50 107 TABLE VI AGEING STABILITY OF AN ALLOY CONTAINING 25% Al, 1% Cu AND 0.05% Mg SLOWLY COOLED (CURVE 2) FROM 380C AND THEN AIR COOLED FROM 250C Tensile Properties Ultimate Tensile EFFECT OF THE TWO STAGE HEAT TREATMENT ON THE TENSILE PROPERTIES OF ALLOYS WITHIN THE SCOPE OF THE INVENTION Heat Treatment Tensile Properties Alloy Alloy Homogenizing Air Cooling Ultimate Reduct- Composition. No. 72 Temperature Temperature Tensile 7e Elonion of Al Cu Mg Bi C C Strength gation Area (lo/inf) 36 25.5 0.98 380 R.T. 38,200 40 70 36 25.5 0.98 380 250 43,800 6O 25.5 3.34 380 RT. 40,800 28 48 40 25.5 3.34 380 250 48.000 26 40 83 25.7 1.03 0.055 380 R.T. 45,800 22 50 83 25.7 1.03 0.055 380 250 52,500 29 45 52 22.1 2.75 0.059 380 R.T. 48.600 22 3O 52 22.1 2.75 0.059 380 250 55,200 22 30 79 26.3 5.1 0.06 380 R.T. 50.300 20 38 79 26.3 5.1 0.06 380 250 59,500 20 30 80 24.9 10.2 0.05 380 R.T. 56,500 l1 l4 TABLE VII Cntinued Heat Treatment Tensile Properties Alloy Alloy Homogenizing Air Cooling Ultimate '7? Reduct- Composition, No. Temperature Temperature Tensile Elonion of Al Cu Mg Bi "C C Strength gation Area (lb/in?) 80 24.9 10.2 0.05 380 250 60,500 9 10 84 25.7 1.01 0.14 0.45 380 RT. 49,400 33 60 84 25.7 1.01 0.14 0.45 380 250 58,000 13 97 25.6 0.134 0.53 380 R.T. 45,900 26 44- 84/85 380 250 64,400 NA. NA. 93 22 1.00 0.136 0.44 380 RT. 46,900 27 45 93 22 1.00 0.136 0.44 380 250 65,400 20 43 94 31 1.02 0.140 0.46 380 RT. 49,300 26 48 94 31 1.02 0.140 0.46 380 250 69,200 18 35 49 12.78 0.80 0.022 380 RT. 42,100 29 64 49 12.78 0.80 0.022 380 250 56,800 21 60 We claim: for incldental impurities, including a two stage cooling 1. A heat treatment for improving the creep resistance of a wrought zinc-aluminum alloy having a zincaluminum eutectoid transformation, including a twostage cooling process comprising:

a. as the first stage of said process, slow cooling the alloy from above its eutectoid temperature and below its solidus or eutectic temperature to an intermediate temperature below the eutectoid temperature whereat the eutectoid transformation is substantially complete, and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, rapidly cooling said alloy to ambient temperature.

2. The heat treatment defined in claim 1 wherein said wrought alloy is a hot worked alloy.

3. The heat treatment defined in claim 1, wherein said intermediate temperature is at least 200C.

' 4. The heat treatment defined in claim 1 wherein said alloy is homogenized at a temperature above its eutectoid temperature and below its solidus or eutectic temperature before the commencement of said slow cooling.

5. The heat treatment as defined in claim 4 wherein said slow cooling of the alloy is effected by allowing a furnace used for said heat treatment to cool at its natural rate.

6. The heat treatment as defined in claim 4 wherein said rapid cooling of said alloy is effected by air cooling the alloy.

7. The heat treatment as defined in claim 4 wherein said slow cooling is at an average rate of less than about 2.0C./min. to the temperature at which said rapid cooling commences.

8. The heat treatment as defined in claim 4 wherein slow cooling comprises cooling said alloy from above its eutectoid temperature and below its solidus temperature to a temperature of 250C. at an average rate of less than about 2C./min.

9. The heat treatment as defined in claim 4 wherein the rapid cooling is commenced at about 250C.

10. The heat treatment for improving the creep resistance of a wrought zinc-aluminum alloy containing 12 to aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except process comprising:

a. as the first stage of said process, homogenizing and slow cooling the alloy from above its eutectoid temperature and below its solidus or eutectic temperature to an intermediate temperature below its eutectoid temperature whereat the eutectoid transformation is substantially complete, and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, rapidly cooling said alloy to ambient temperature.

11. A heat treatment for improving the creep resistance of a wrought zinc-aluminum alloy containing 18 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities, including a two-stage cooling process comprising:

a. as the first stage of said process homogenizing and slow cooling the alloy from above its eutectoid temperature and below its solidus temperature to an intermediate temperature below its eutectoid temperature whereat the eutectoid transformation is substantially complete, and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, rapidly cooling said alloy to ambient temperature.

12. The heat treatment as defined in claim 11 wherein said allow is homogenized at a temperature such that said alloy is substantially in the a phase.

13. The heat treatment defined in claim 11 wherein said wrought alloy is a hot worked alloy.

14. A heat treatment for improving the creep resistance of wrought zinc-aluminum alloys containing 18 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities, said treatment including a two-stage cooling process comprising:

a. as the first stage of said process, homogenizing the alloy at about 380C, and slow cooling the alloy from the homogenization temperature to an intermediate temperature of about 250C. at an average rate of cooling of about lC./min. and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, air cooling said alloy from about 250C. to ambient temperature.

l =8 l l 

1. A HEAT TREATMENT FOR PROVIDING THE CREEP RESISTANCE OF A WROUGHT ZINC-ALUMINUM ALLOY HAVING A ZINC-ALUMINUM EUTECCOMPRISING: COMPRISING: A. AS THE FIRST STAGE OF SAID PROCESS, SLOW COOLING THE ALLOY FROM ABOVE ITS EUTECTIOD TEMPERATURE WHEREAT THE EUTECDUS OR EUTECTIC TEMPERATURE AND BELOW ITS SOLITURE BELOW THE EUTECTOID TEMPERATURE WHEREAT EUTECTIOD TRANSFORMATION IS SUBSTANIALLY COMPLETE, AND SUBSEQUENTLY B. AS THE SECOND STAGE OF SAID PROCESS, UPON ATTAINMENT OF SAID INTERMEDIATE TEMPERATURE, RAPIDLY COOLING SAID ALLOY TO AMBIENT TEMPERATURE.
 2. The heat treatment defined in claim 1 wherein said wrought alloy is a hot worked alloy.
 3. The heat treatment defined in claim 1, wherein said intermediate temperature is at least 200*C.
 4. The heat treatment defined in claim 1 wherein said alloy is homogenized at a temperature above its eutectoid temperature and below its solidus or eutectic temperature before the commencement of said slow cooling.
 5. The heat treatment as defined in claim 4 wherein said slow cooling of the alloy is effected by allowing a furnace used for said heat treatment to cool at its natural rate.
 6. The heat treatment as defined in claim 4 wherein said rapid cooling of said alloy is effected by air cooling the alloy.
 7. The heat treatment as defined in claim 4 wherein said slow cooling is at an average rate of less than about 2.0*C./min. to the temperature at which said rapid cooling commences.
 8. The heat treatment as defined in claim 4 wherein slow cooling comprises cooling said alloy from above its eutectoid temperature and below its solidus temperature to a temperature of 250*C. at an average rate of less than about 2*C./min.
 9. The heat treatment as defined in claim 4 wherein the rapid cooling is commenced at about 250*C.
 10. The heat treatment for improving the creep resistance oF a wrought zinc-aluminum alloy containing 12 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities, including a two stage cooling process comprising: a. as the first stage of said process, homogenizing and slow cooling the alloy from above its eutectoid temperature and below its solidus or eutectic temperature to an intermediate temperature below its eutectoid temperature whereat the eutectoid transformation is substantially complete, and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, rapidly cooling said alloy to ambient temperature.
 11. A heat treatment for improving the creep resistance of a wrought zinc-aluminum alloy containing 18 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities, including a two-stage cooling process comprising: a. as the first stage of said process homogenizing and slow cooling the alloy from above its eutectoid temperature and below its solidus temperature to an intermediate temperature below its eutectoid temperature whereat the eutectoid transformation is substantially complete, and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, rapidly cooling said alloy to ambient temperature.
 12. The heat treatment as defined in claim 11 wherein said allow is homogenized at a temperature such that said alloy is substantially in the Alpha '' phase.
 13. The heat treatment defined in claim 11 wherein said wrought alloy is a hot worked alloy.
 14. A heat treatment for improving the creep resistance of wrought zinc-aluminum alloys containing 18 to 30% aluminum, 0 to 10% copper, 0 to 1% magnesium, 0 to 3% bismuth, the balance being zinc except for incidental impurities, said treatment including a two-stage cooling process comprising: a. as the first stage of said process, homogenizing the alloy at about 380*C., and slow cooling the alloy from the homogenization temperature to an intermediate temperature of about 250*C. at an average rate of cooling of about 1*C./min. and subsequently b. as the second stage of said process, upon attainment of said intermediate temperature, air cooling said alloy from about 250*C. to ambient temperature. 